Carbocyanine dyes and method of preparing them



Patented Nov. 7, 1933 STATES CARBOCYANINE DYES AND METHOD OF PREPARING THEM.

Leslie G. S. Brooker, Rochester, N. Y., assignor' to Eastman Kodak Company,.Rochestcr, N. Y.

a corporation of New York No Drawing. Application MarchS, 1931 Serial No. 520,461 r Claims.

This invention relates to a new composition of matter, and particularly to a new class of photographic sensitizing dyes, and a method for their preparation.

As ordinary photographic emulsions are generally indifferent to those colors of the spectrum of longer wave lengths, various types of sensitizing dyes have been suggested for incorporation in photographic emulsions, or for employing over emulsions as overcoatings so as to increase the spectral sensitiveness thereof. Many cyanine dyes have been described for this purpose containing the benzothiazole nucleus, whilst classes .or" these photo-sensitizing dyes containing the naphthothiazole nucleus are described in my co- H HO pending applications of Serial No. 337,177, filed February 2, 1929; Nos. 435,104 and 435,105, filed -March 12, 1930; and No. 437,017, filed March 19, 1930, whilst those containing the thiazoline nucleus are described in my co-pending application Serial No. 460,548 filed June 11, 1930 and those containing the thiazole nucleus are described in my co-pending application Serial No. 548,025 filed June 30, 1931.

An object of the present invention is to pro-' vide a process for the preparation of photosensitizing dyes of the thiocarbocyanine series containing benzothiazole nuclei and having aryl (or substituted aryl) substituents attached to the central carbon atom of the three-carbon chain.

A further object of this invention is to provide such photo-sensitizing dyes as may be added to photographic emulsions or coated thereon as an over-coating whereby the spectral sensitivity of the photographic emulsion is increased. Other objects willhereinafter appear.

I have found that dyes having good photosensitizing properties may be prepared by the treatment of a l-methylbenzothiazole alkyl quaternary salt with an ortho-ester of an aro-.

matic carboxylic acid. Ortho-esters of arcmatic carboxylic acids which have been found suitable include such esters as trimethyl orthobenzoate and trimethyl ortho-p-toluate.

Q The aromatic acids whose trialkyl orthoesters are employed may be further described as possessing an aryl group directly linked to the carbon atom of the carboxyl group. This aryl g'roup'may be unsubstituted, as for eXam-. ple'the phenyl group, or substituted, as for BX'. ample the p-tolyl group. In this latter example the aryl group (phenyl) is seento be substituted by an alkyl group (methyl) but substituents other than alkyl groups may be employed.

The reaction of a l-methylbenzothiazole alkyl quaternary salt with oneof these ortho-esters of a carboxylic acid in the presence of pyridine may be'written graphically as follows:

In the above structures Rfrepresents an alkyl group andX an acid radicaL. A stands forthe aryl (or substituted aryl), group to be introduced intothe three-carbon chainrof the thiocarbocyanine dye and'OBf and 0R v represent the alkoxy groupings of the ortho-ester and these may be similar or dissimilar. The dye molecule is numbered as shown, so that a compound of this type may be described as a 2:2-dialykyl-8-'aryl (unsubstituted or substituted) -thiocarbocyanine salt.

In actual practice it is frequently desirable to carry out the condensation with an acid radical such as the p-toluene-sulfonate,- which confers a comparatively high degree of solubility on the lnethylbenzothiazole alkyl quaternary salt and on the dye. The latter is thenconveniently isolated as a less soluble salt by double-decomposition in the ordinary way, such salts as ammoniumor potassium bromide or iodide being employed for thispurpose, usually in, aqueous solu-' tion.

The -CI-I= group is called the methenyl group and therefore the chain'CH=CH-CI-I= may be called a tri-methenyl chain, and the chain may similarly be called a tri-methenyl chain in which the hydrogen attached to the central carbon atom has been replaced by the group denoted by the letter A.

The nomenclature and numbering of the above classes of compounds and intermediates are in accord with present usage and also the system used in my co-pending applications given above. The numbering, of course, is arbitrary and is given for the guidance of those skilled in the art in order that there be no question as to the identity of a compound covered by my invention.

The methods for the formation of the l-methylbenzothiazoles (used in the preparation of the photographic sensitizing dyes herein described) are given in the literature. One general method consists of the conversion of the acetyl compound of an aromatic primary amine, such as p-toluidine, into the thio-acetyl compound by the action of phosphorous pentasulfide. This thio-acetyl compound is'then dissolved in aqueous caustic alkali solution and oxidized with a cold aqueous solution of alkali ferricyanide whereupon the base is producedand may be extracted with ether.

The preparation of some of the ortho-esters of the aromatic carboxylic acids have not, however, been described in the literature and I will give here, therefore, one method for their preparation.

.. These ortho-esters are prepared by taking the nitrile of the corresponding acid as the starting material. The nitrile (one mol.) is dissolved in dry absolute ethyl ether together with an alcohol (1 mol.) such as methyl or ethyl alcohol, all of ,the reactants being as free from water as possi- 1e and dry hydrochloric, acid gas (rather more than 1 mol.) is led in, using good cooling throughout the operation. The whole is kept cold and.

the imino-ether hydrochloride separates and is removed by filtration when crystallization appears to be complete. It is then carefully washed with dry absolute ethyl ether and dried in a vacuum over solid caustic potash until free from. all excess of hydrochloric acid. The dry imino-ether hydrochloride is then decomposed at room temperature with an alcohol, which again must be as anhydrous as possible, taking roughly 2 to 3 cc. of the alcohol for every gram of the hydrochloride. The latter dissolves more or less completely-and the formation of the ortho-ester is accompanied by the separation of ammonium chloride, the formation of which may be used as an in dication of the course of the reaction;

The reaction vessel is tightly stoppered and allowed to stand for several days or even several weeks until the reactionis apparently complete. The insoluble material is then filtered off and the excess alcohol is removed by distillation, filtering again if necessary. The ester is finally distilled over, using diminished pressure as a rule, and is purified by refractionation. Thus benzonitrile on treatment with ethyl alcohol and hydrochloric acid gas in the manner described above yields benzo-imino-ethyl-ether hydrochloride, and this, when subjected to the action of ethyl alcohol in the manner described, yields triethylorthobenzoate. Similarly, p-tolylcyanide or 'p-tolunitrile yields trimethyl ortho-p-toluate. The ortho-esters' of other aromatic acids may be prepared in a similar manner, and mixed esters may be made by taking two different alcohols for the two separate steps of the reaction.

I will now give several examples for preparing various dyes referred to, but it will be understood that they are merely representative of a great number of reactions in which the proportions given and equivalents used may vary in accord with the particular type of dye required. My invention, therefore, will not be restricted thereby except as indicated in the appended claims.

The 1-methylbenzothiazole used in any of the described condensations may be unsubstituted in the benzene nucleus or it may be substituted therein, as for example by a fused-on benzene nucleus, as described in my co-pending application Serial No. 435,104.

EXAMPLE 1 2:2-dimethyl-8phenylthiocarbocyzmine bromide 16.8 g. (1 mol.) l-methylbenzothiazole methop-toluene-sulfonate was refluxed for one hour with 11.2 g. (1 mol.) triethyl ortho-benzoate and cc. pyridine. The hot solution was then treated with 20 g. potassium bromide dissolved in cc. hot water. A solid felted mass oi crystals was formed but on standing, with occasional stirring, this all became granular. The crystals were filtered oil, well washed with Water and then with acetone and dried. 7.5 g. of dull greenish-brown crystals were obtained and these, re-crystallized from methyl alcohol yielded beautiful large bronze-like crystals.

EXAMPLE 2 2:2 -dethyl-8-phenylthiocarbocg anine bromide 35 g. (1 mol.) l-methylbenzothiazole etho-ptoluene-sulfonate was refluxed for one hour with 22.4 g. (1 mol.) triethyl ortho-cenzoate' (prepared from benzonitrile) and 25 cc. dry pyridine. A solution of 30 potassium bromide in cc. of hot water was added and the whole allowed to stand overnight. The crystals were filtered off and washed successively with ether, cold acetone, cold water and again cold acetone and dried. at 110. Weight 16.1 g. The dye was recrystallized from methyl alcohol, in which it gave a bluishred solution, and formed beautiful large prisms, with well shaped faces which reflected greenishblue light.

EXAMPLE 3 2:2 :5 :5 tetmmethyl-8-phenylthiocarbocyanine bromide 5 g. (1 mol.) 1:5-dimethylbenzothiazole was heated overnight at 100-110 C. with 5.? g. (1 mol.) methyl p-toluenesulfonate, and the product refluxed for one hour with 6.9 g. ('1 mol.) triethyl ortho-benzoate and 15 cc. pyridine. The reaction mixture was then treated with a solution of i0 potassium bromide in 50 cc. hot water, when a bulky precipitate formed which rapidly became crystalline on stirring. When the wholewas cold the crystals were filtered ofi, well washed with cold water and cold acetone and dried. flex were obtained, which yielded minute dark greenish-brown crystals on recrystallization from methyl alcohol. The solution in this solvent was blush red.

EXAMPLE 4 2:2diethyZ-S-p-tolylthiocarbocyanine iodide 14 g. (1 mol.) i-methylbenzothiazole etho-ptoluene-sulionate was refluxed i or one hour with 15.? g. (2 mols.) trimethyl ortho-p-toluate (prepared from p-tolunitrile and 45 cc. dry pyridine. On treating with a solution of 26 g. potassium iodide in 100 cc. water and allowing to stand- 5.1 g. of dark crystals with a bronze reovernight the dye was precipitated and'was obtained after filtering and washing as a dullgreyish mass with coppery crystals and also blue crystals visible as well. Weight when-dried at 110 3.9 g. l v

10.4 g. of such a mixture as the above was boiled out with successive portions of methyl alcohol, filtering hot each time, until only a small amount of green crystalline material remained in the residue. The filtrate, on cooling slowly, deposited large green crystals of the dye sought for, as well as smaller crystals of an impurity. The latter were best removed by flotation in carbon tetrachloride, when the large green crystals floated on the surface and the impurity was suspended in the body of the liquid, and could be drawn off with a pipette. 1.76 g. of green crystals were so obtained, fairly pure, and were obtained quite pure (1.255 g.) on crystallizing from '75 cc. methyl alcohol. The dye gave a bluish-red solution in methyl alcohol.

By substituting various unsubstituted or sub stituted aryl groups in the position in the structural formula of the dye occupied by the letter A, and alkyl groups in the two positions occupied by the letter R many sensitizing dyes may be obtained. The letter X indicates any suitable anion such, for example, as a halide, p-toluenesulfonate, alkylosulphonate, nitrate, acetate, perchlorate, or, in fact, any salt forming anion that will not render the dye too sparingly soluble or deleteriously afifect the photographic sensitizing properties of the dye.

It is apparent from a thorough consideration of the above description that any thiocarbocyanine dye prepared from a l-methylbenzothiazole unsubstituted or substituted in which the 8 position is substituted by an aryl group (itself unsubstituted or substituted), will come within the scope of this invention whatever may be the radicals occupying the other substitutable positions of the structure.

Furthermore, it will also be apparent that the genus claimed hereinafter not only includes the foregoing but also includes any carbocyanine dye containing two of the five membered rings linked together by a tri-methenyl chain, the hydrogen attached to the central carbon atom of which is substituted by an aryl or substituted aryl group such as, for example, the 8-substituted thiocarbocyanines derived from rnu-rnethylnaphthothiazole described in my application #435,104, the '7-substituted thiazolinocarbocyanines derived from 2-rnethylthiazoline described in my application #4605 18 and the T-substituted thiazolocarbocyanines derived from 2-methylthiazole described in my application #548,026, In the claims, therefore where (1) an aryl group or (2) an aromatic carboxylic acid is referred to it is definitely intended that such shall include, re-

spectively, (1) both an unsubstituted and a substituted aryl group or (2) an aromatic carboxylic acid having an unsubstituted aryl group directly linked to the carbon atom of the carboxyl group .i and an aromatic carboxylic acid having a suba rived from benzene; this term is not intended to include aryl substituted aliphatic groupingsthese beingcovered in my copending application Serial No. 520,460; the aryl group may, of course, be substituted by. groups which contain elements other than carbon and hydrogen; in any event the aryl radical is one which is linked to the trimethenyl chain by attachment to a nuclear carbon atom of the hydrocarbonring of the aryl radical. The aromatic acids, referred to in the claims, are limited to those containing such an aryl radical.

While the principal use'now known for these new chemical compounds is for sensitizing photographic emulsions it may well be that other uses therefore will be discovered from time to time and consequently it is not intended that the foregoing description of the sensitizing properties of these dyes shall in any way limit the scope of the claims appended hereto. The combination of the dyes with a photographic emulsion is claimed in my co-pending application Serial No. 505,984 filed December 31, 1930.

What I claim as my invention and desire to be secured by Letters Patent of the United States is:

1. A thiocarbocyanine dye of the following structure structure in which D equals a phenylene group, R equals an alkyl radical, X equals an acid radical and A equals an aryl radical of the benzene series.

3. A thiocarbocyanine salt substituted in the 8 position with an aryl radical of the benzene series.

4. A 2:2-dialkyl-8-arylthiocarbocyanine salt in which the aryl radical belongs to the benzene series.

5. A 2 :2 -dialkyl-8-phenylthiocarbocyanine salt.

6. A 2:2 dialkyl 8 phenylthiocarbocyanine halide.

7. A 2:2 dialkyl 8 phenylthiocarbocyanine bromide.

8. 2:2 dimethyl 8 phenylthiocarbocyanine bromide.

- 9. 2:2'-diethyl-8-phenylthiocarbocyanine bromide.

10. 2:2 -diethyl-8-p-tolylthiocarbocyanine bromide.

11. A 2 :2 diethyl 8 p-tolylthiocarbocyanine salt.

12. The process of preparing carbocyanine dyes which comprises condensing, under the influence of heat, one molecular proportion of an orthoester of an aromatic rnonobasic carboxylic acid of the benzene series with two molecular proportions of a compound having the structure which comprises condensing, under the influence of heat, a l-methylbenzothiazole alkyl quaternary salt with a trialkyl ortho-ester of benzoic acid.

15. The process of preparing carbocyanines which comprises condensing, under the influence of heat, a l-methylbenzothiazole alkyl quaternary salt with triethyl ortho-benzoate;

LESLIE G. S. BRIOOKER. 

